1. Technical Field
This invention relates to a modulator for modulating digital signals, and to a demodulator.
2. Discussion of Related Art
It is known, for example from the journal "Der Fernmeldeingenieur", 8 and 9/1993, Verlag fur Wissenschaft und Leben Georg Heidecker GmbH, Erlangen, pages 51 to 56, to transmit digital signals by the single-carrier method or by the multicarrier method. Both methods use one carrier frequency per transmission channel or per subchannel of predetermined bandwidth to transmit digital signals. Quadrature amplitude modulation (QAM), for example, is applied to the digital signals, and the result is assigned to the carrier frequency of the respective transmission channel.
In an article entitled "Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come", IEEE Communications Magazine, May 1990, pages 5 to 14, the problems associated with the overlapping of spectra of a transmission channel of predetermined bandwidth and a neighboring transmission channel are considered. If the overlap of the spectra is such that their -3 dB frequencies lie one above the other, the spectra of the individual transmission channels can still be detected at the receiver end by means of suitable filters.
The transmission of digital signals is thus limited by the bandwidth of a transmission channel. The alignment of transmission channels or subchannels in terms of frequency is limited by the overlap of the spectra into neighboring channels.
It is therefore an object of the invention to increase the transmission capacity of digital signals without changing the bandwidth of a transmission channel.